When the fat-blocker Alli hit pharmacy shelves last June, hopeful consumers stampeded for the first government-approved over-the-counter weight-loss drug. But it didn’t take long for the
troublesome side effects such as not being able to control your bowels
, and the lack of real weight loss for some, to convince many dieters that Alli wasn’t the sole answer to their weight problems.

In addition to Alli, there are about six prescription weight-loss drugs on the market. Some, like Alli, block the absorption of fat in the body. Others work in the brain to suppress appetite. But the reality is, no matter how many weight-loss pills you take, they don’t work by themselves.

“The pills we currently have don’t seem to be terribly effective for long-term weight loss,” says registered dietitian Anne Fletcher of Mankato, Minn.“When people go off the pills, many put weight back on.”

Still, the drug industry and many dieters remain convinced that a pill is the answer to the nation's growing obesity epidemic. Several new
prescription diet pills are in late stage trials
, including the experimental drug lorcaserin, a variation of Fen-phen
which was pulled for causing heart problems
. At least 30 companies are developing weight-loss drugs, with experts estimating that in the next few years there will be 10 to 15 medications meant to help dieters in different ways. The pharmaceutical research firm Decision Resources projects that the obesity drug market will grow in the U.S. from $222 million in 2006 to nearly $2 billion by 2016.

When combined with diet and exercise, anti-obesity drugs can help someone lose about 5 to 10 percent more weight, research suggests. But because the various pills affect the body in different ways, it’s possible that certain people may be better suited to using one type of pill over another.

For example, a study looking at the differences between sibutramine (Meridia) and orlistat (Xenical and Alli), found that greater weight loss with orlistat occurred in people who could be described as “conscientious,” that is, their personalities were more order-oriented or deliberate. Therefore, carefully monitoring fat intake works for them. People who have a hard time restraining their eating habits saw a greater impact with sibutramine — which affects serotonin in the brain and makes you feel full sooner when eating.

If at first, you don't succeed ...In general, the biggest mistake people make in taking diet pills is looking at them as magic bullets.

"People cannot expect a medication to do it for them,” says Gary Foster, director of the Center for Obesity Research and Education at Temple University in Philadelphia. “It’s a 50/50 partnership with the pill being half the equation. Exercise, what we eat and our lifestyle habits … are critical to long-term success.”

When clients request to use a weight loss medication, Fletcher, author of the book “Thin for Life,” tells them to think of it as “one leg of a four-legged stool.”

“One leg is physical activity; one is diet; one is behavior change and the final leg can be diet pills,” she says. “Take out any of the legs that support the diet pill and the stool will fall over.”

From her research Fletcher found that people who lost significant amounts of weight often had made multiple efforts to lose weight before they realized long-term success.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

“When we try to change any behavior it takes most of us a few attempts to get it right,” she says.

Side effectsThere’s no real data on how safe the pills are long-term, especially for young people. However, the Food and Drug Administration has only approved them for adults for up to two years of continuous use. According to an analysis of 30 trials done on adults taking anti-obesity drugs for one to four years, about 30 to 40 percent stopped taking them after a year, although it wasn't clear why.

In addition, side effects are common. Sibutramine can increase blood pressure and heart rate in some patients. Orlistat, which alters the absorption of fat in food, can have embarrassing intestinal side effects. Rimonabant (used in Europe, but awaiting approval in this country) may, for some users, cause nausea, anxiety, depression and insomnia.

However, for the obese — people with a body-mass index of 30 or greater— the benefits of prescription diet drugs can outweight the risks, experts believe.

“The people most successful are those who are ready to make a serious commitment to losing weight and willing to take responsibility for their actions and for change," says Pat Baird, a dietitian and nutrition consultant for GlaxoSmithKline’s Consumer Healthcare division, who serves as an online moderator for people using Alli. She talks with 25 people a day, fielding questions about weight loss, the program and the pill.

Baird concedes that the side effects are part of the reason Alli works for some people. When someone eats more than the prescribed 15 grams of fat per meal, they can experience the not-so-pleasant digestive problems. "It helps keep people honest," she says.

Susan Moores, R.D., is a nutrition consultant and spokesperson for The American Dietetic Association